1. Field of the Invention
The present invention relates generally to a method of calculating a motor control voltage and a motor control device using the method, and more particularly, to a method of detecting the rotational speed of a motor to calculate a control voltage on the basis of the rotational speed detected and a motor control device for controlling the motor at constant speed on the basis of the results of the calculation.
2. Description of the Prior Art
As control methods for keeping the rotational speed of a motor constant, proportional control has been conventionally known. The proportional control is a control method of detecting the actual rotational speed of a motor and supplying a control voltage proportional to the difference between the actual rotational speed and the target speed.
However, the conventional proportional control has the disadvantage in that it does not rapidly follow the change in speed. More specifically, in a case where the actual rotational speed is lower than the target speed, if it is attempted to increase the rotational speed of the motor to the target speed, it takes relatively long for the rotational speed of the motor to reach the target speed. In particular, the higher the target speed is, the longer it takes for the rotational speed of the motor to reach the target speed.
The foregoing will be described more concretely.
An equation of motion at the time of application of a voltage V to the motor is generally as follows: ##EQU1## where R.sub.a : armature resistance [.OMEGA.]
K.sub.T : torque constant [kgm/A] PA1 K.sub.e : induced voltage constant [V/rpm] PA1 I.sub.0 : no-load current [A] PA1 GD.sup.2 : moment of inertia by load and motor [kgm.sup.2 ] PA1 T.sub.BL : sliding load [kgm].
This equation will be solved for n. If n=N.sub.P in the case of t=0, n is as follows: ##EQU2## and ##EQU3##
From this equation, acceleration a in a case where the sampled speed is N.sub.S is given by the following equation with the substitution N.sub.P =N.sub.S and t=0: ##EQU4##
Let N be the target speed, N.sub.S be the sampled speed and .DELTA.N be the difference therebetween. In this case, acceleration a in a case where a voltage V=K.DELTA.N=K(N-N.sub.S) is applied is as follows by the conventional proportional control with the substitution of V=K.DELTA.N, N.sub.S =N-.DELTA.N in the equation (4): ##EQU5##
This equation shows that even if .DELTA.N is the same value, the acceleration a is small if the target speed N is large while being large if N is small.